Direct drive wind turbine

ABSTRACT

A wind turbine is provided that minimizes the size of the drive train and nacelle while maintaining the power electronics and transformer at the top of the tower. The turbine includes a direct drive generator having an integrated disk brake positioned radially inside the stator while minimizing the potential for contamination. The turbine further includes a means for mounting a transformer below the nacelle within the tower.

FEDERAL RESEARCH STATEMENT

This invention was made with Government support under contractDE-FC36-03GO13131 awarded by the Department of Energy. The Governmenthas certain rights in this invention.

FIELD OF INVENTION

This disclosure relates generally to wind turbine and especially to windturbines with a direct connection between the turbine and the electricalgenerator.

BACKGROUND OF THE INVENTION

The wind has historically been one of the most widely used naturalresources to provide the energy necessary to power our needs. As thedemand for energy has increased and the supplies of fossil dwindled,resulting there has been a renewed look by electrical utility companiesat alternative methods for producing electrical power. One method ofelectrical production involves the harnessing of the wind by a windturbine to drive an electrical generator.

Wind turbines typically involve using a series of blades fixed to thetop of a tower to rotate about a horizontal axis. The blades have anaerodynamic shape such that when a wind blows across the surface of theblade, a lift force is generated causing the series of blade to rotate ashaft about an axis. The shaft is connected, typically via a gearingarrangement, to an electrical generator located in a structure called anacelle which is positioned behind the blades. The gear box converts therotation of the blades into a speed usable by the generator to produceelectricity at a frequency that is proper for the electrical grid it isproviding power.

The nacelle houses a number of components which are needed in modernhigh capacity wind turbines. In addition to the aforementioned gear boxand generator, other components include a yaw drive which rotates thewind turbine, various controllers, and a brake that is used to slow thegenerator. Since it is desirable to keep the nacelle as small aspossible, and given the number of relatively large pieces of equipmentwhich must be located in the nacelle, space becomes very valuable. Thisoften results in difficulties in both manufacturing the wind turbine andin conducting maintenance operations in the nacelle once the windturbine is installed.

Accordingly, it is considered desirable to provide a wind turbine whichminimizes the size of the nacelle while providing adequate accessibilityto components during maintenance operations.

SUMMARY OF THE INVENTION

A wind turbine is provided that includes a nacelle with a rotor hubadjacent thereto. The turbine has a main shaft coupled to the hub andthe nacelle. A generator is coupled to the shaft between the nacelle andthe hub, wherein the generator includes rotor adjacent to the shaft.Also a stator is positioned adjacent to and radially outward from therotor and, a brake is coupled to the generator and the shaft, such thatthe brake is positioned radially inward from said stator.

A wind turbine is also provided including a tower having a yaw bearingattached at one end. A nacelle having a bedplate is connected to the yawbearing and a transformer is positioned within the tower opposite thenacelle. In a first alternate embodiment, the transformer is suspendedby a chain. In a second alternate embodiment, the transformer issuspended in a viscous fluid in a container connected to the tower.

A wind turbine is further provided having a nacelle and a blade rotorhub adjacent to the nacelle. A main shaft is coupled to the blade rotorhub and the nacelle. Also a generator is coupled to the shaft betweenthe nacelle and the hub, the generator having a housing containing agenerator rotor adjacent to the shaft and a stator positioned adjacentto and radially outward from said rotor. A cylindrical roller bearing iscoupled between the shaft and the housing adjacent to the nacelle. Asecond bearing is coupled between the shaft and the housing adjacent tothe hub.

Also, a method for transferring electrical power from a wind turbine isprovided including the steps of rotating blades using wind. Rotating agenerator and generating electricity with the generator. Supporting thegenerator with a tower and suspending a transformer adjacent to thegenerator. Damping the movement of the tower by contacting thetransformer and transmitting the electricity through the transformer.

The above discussed and other features will be appreciated andunderstood by those skilled in the art from the following detaileddescription and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, which are meant to be exemplary and notlimiting, and wherein like elements are numbered alike:

FIG. 1 is a plan view illustrating a direct drive wind turbine of thepresent invention;

FIG. 2 is a side plan view of the wind turbine of FIG. 1;

FIG. 3 is a side plan view, partially in section of the wind turbine ofFIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT

Electrical power may be generated by many different methods. The mostcommon methods involve the boiling of water using fossil or nuclearbased fuels. The steam produced by the boiling is used to rotate aturbine that drives an electrical generator to create the electricalpower. While these common methods are very efficient, they also haveundesirable side effects, such as the production of toxic pollutants, orthe rely on a dwindling natural resource. One alternate method ofcreating electrical power is to harness a renewable natural resourcesuch as the wind to be a driving force to rotate the electricalgenerator to produce the electricity.

Referring to FIG. 1 and FIG. 2, a wind turbine 10 capable of generatingelectrical power in the 100 kw to 2000 kW range is shown. The windturbine 10 is includes a tower 12 which is anchored to the ground bymeans of a bolted connection to a steel and concrete foundation. On theopposing end of the tower 12, the nacelle 14 is mounted to rotate aboutthe tower 12 to allow the nose cone 16 and the plurality of blades 18 toface into the wind. As will be described in more detail herein, agenerator 20 is positioned between the nose cone 16 and the nacellewhich allows the size of the nacelle to be minimized while stillallowing all the necessary power electronics and controls to locatedeither in the nacelle 14 itself, or adjacent the top of the tower 12.

Typically for this size turbine, the tower 12 is between 20 and 100meters in height and constructed of tapered tubular steel of up to 4meter diameter at the ground and 1–2 meter diameter at the top. Thetapered tubular steel tower is constructed in sections to facilitate thetransportation and assembly of the wind turbine 10 at its point of use.Alternatively, the tower may be made from a lattice structure or fromconcrete sections. In the preferred embodiment, there are three turbineblades 18 of 10–45 meters in length that equally spaced around the nosecone 16. While the blades may be made of any suitable material,typically a glass fiber reinforced plastic or epoxy is used to reduceweight while still providing the necessary mechanical strength requiredto withstand the wind loads. To reduce the complexity of the windturbine 10 the blades 18 are preferably of a fixed pitch type, thoughvariable pitch blades could also be used as well.

Turning to FIG. 3, the nacelle 14 and generator 20 will be described inmore detail. The nacelle 14 has a bedplate 22 which forms the floor ofthe nacelle 14 and a cover 15 which encloses the bedplate 22. Thebedplate 22 is mounted to a yaw bearing 24 that is mounted a top thetower 12. The yaw bearing 24 allows the nacelle 14 to rotate relative tothe tower 12 to allow the blades 18 to orient correctly relate to thewind ensuring maximum energy production. A yaw drive 26 mounted insidethe nacelle 14 drives a pinion 28 which interacts with gear teeth 35 onthe outer race of yaw bearing 24 to provide the necessary force torotate the structure. The controller 62 receives information on the winddirection from a wind sensor 66 which activates the yaw drive 26. Thesafety system of the wind turbine uses an anemometer 27. Whenever thewind speed exceeds a pre-determined safe value, the wind turbine shutsdown. A typical wind speed for shut down is 25 meters/second. Since itis desirable to transfer the electrical power from the nacelle 14 to thegrid at a high voltage to reduce the required cable size, in thepreferred embodiment, a transformer 30 is suspended below the bedplate22 inside the tower 12 by a chain 29. It should be appreciated that thetransformer 30 may be mounted to the bedplate 22 by any suitable means,preferably a means that allows some flexure to compensate for vibratorymovement of the wind turbine 10.

By arranging the transformer beneath the nacelle 14 inside the tower 12,the transformer 30 is allowed to rotate with the nacelle 14 whilereducing the required size of the nacelle. Preferably, the transformer30 will also have an opening 31 in the center to allow access to thenacelle 14 by maintenance personnel from within the tower 12. In analternative embodiment, the transformer is sized to allow periodiccontact between the transformer 30 and the tower 12 which will act tomechanically damp any oscillations of the tower which may occur. Thetransformer 30 may be of any electrical type suitable for a windturbine, including both the dry-type and oil-filled, 3-phase Wye or3-phase delta, high voltage or low voltage. In another alternateembodiment, the transformer is of a rectangular shape, and placed to oneside in the tower 12 to allow access to the nacelle 14 by maintenancepersonnel. In another alternate embodiment, the transformer is suspendedin a bath of viscous fluid that is attached to the tower 12 to provideviscous damping of any oscillations of the tower 12.

The transformer 30 connects via cable 33 to the power electronics 32mounted inside the nacelle 14, typically on the cover 15. As will bedescribed in more detail below, the power electronics 32 receiveselectricity from the generator 20 and converts the variable frequencyelectricity to match the frequency required by the electrical grid thatwind turbine 10 is connected. For a typical application, the generator20 produces at a frequency between 10–30 Hz and the power electronics 32use conventional means to produce the frequency of the electrical grid,typically 50 Hz or 60 Hz. The power electronics 32 may utilize anintermediate conversion of alternating current (“AC”) power from thegenerator to direct current (“DC”) power before converting to AC powerat the grid frequency. Power throughput and terminal power factor areadjustable via controller commands (not shown).

The generator 20 includes a housing 34 which is mounted to the bedplate22. The housing 34 connects to a main drive shaft 36 through frontbearing 38 and rear bearing 40. In the preferred embodiment, the frontbear 38 is a double-tapered roller bearing sized to carry a majoritybending moment and axial thrust generated by the blades 18.Alternatively, the front bearing 38 may be a crossed roller bearing or athree row type roller bearing. If the bearing 38 was required to supportlarge bending moments by itself, the distance between the rollers wouldbe large requiring a larger drive shaft 36 which would dramaticallyincrease the cost of the wind turbine 10. To make this arrangement morecost effective, a second rear bearing 40 is used to assist the frontbearing 38 in carrying the bending moment. Preferably, the rear bearing40 is a cylindrical type bearing.

By properly spacing the bearings 38, 40 the forces generated by theblades 18 can be carried while minimizing the size of the drive shaft36. In the preferred embodiment, the front bearing 38 and the rearbearing 40 are spaced apart a distance equal to the diameter of thedrive shaft 36. Between the bearings 38, 40, the generator rotor 52 ismounted via a hub 54. The rotor 52 rotates inside the housing 34adjacent to the stator 56. The rotor has electrical coils which areenergized with direct current, creating a magnetic field. As the shaft36 is driven by the blades 18, the rotor 52 rotates a magnetic fieldwhich induces electrical current in the stator 56. The electricalcurrent flows from the stator 56 through cable 58 to power electronics32 in the nacelle 14.

In order to provide electric current to the generator rotor 56, a slipring assembly 42 is provided at the end of the drive shaft. The slipring assembly 42 is mounted to the bedplate 22 by strut 43, whichprevents rotation of the housing of the slip ring assembly 42 relativeto the shaft 44. Mounted on the slip ring assembly is a speed sensor 60,which measures the rotational speed of the shaft 44. Further along theshaft, a disk 46 is mounted to the shaft 36 adjacent to the housing 34.For reasons that will be made clearer herein, the disk 46 interacts witha brake 48 which is used to slow the turbine blades. The brake 48 may beof any conventional type such as caliper actuated by hydraulic,pneumatic or electrical pressure. In the preferred embodiment, the disk46 and brake 48 are positioned in a recess 50 in the housing 34 Theshaft 36 terminates in a flange 44 to which the nose cone 16 mounts.

In operation, the turbine controller 62 receives information from winddirection sensor 00 indicating the direction of the wind. If the blades18 are not oriented correctly with the respect to the wind, the windturbine controller 62 activates and powers a yaw drive 26 powers a motorwhich drives the gear 28 to rotate the nacelle 14 and blades 18 to thecorrect position. If there is sufficient wind to drive the blades 18,typically 4–25 meters per second, the rotation of the blades 18 willturn the shaft 36 and the rotor 52 to generate the electrical current asdescribed herein above. The wind turbine controller 62 periodicallychecks the wind direction, typically once multiple times per second.

Since over speeding of the wind turbine 10 due to excessively high windspeeds could damage the generator, it is desirable to have a means forslowing down the blades 18 and the shaft 36. Unlike in a variable pitchturbine which has blades that can be rotated to reduce the amount oflift generated on the blades, the blades 18 of the preferred embodimentare of a fixed pitch. The aerodynamic design of the fixed-pitch bladescauses stall at higher wind speeds to reduce lift, provided therotational speed of the blade rotor is limited. The speed is controlledunder normal conditions by adjusting the generator torque using thepower converter or the rotor current. In the event that an over speedcondition occurs, two independent braking systems are normally applied,both with the capability to stop the rotor. The first system is anelectrical dynamic brake, which uses a resistor to dump energy andcreate a high torque on the generator 20. The second system uses amechanical brake 48 to slow the blades 18. In the event that an overspeed condition is detected by speed sensor 60 or alternatively by arotary encoder (not shown) located adjacent the slip rings downwind ofthe main shaft, the caliper 49 on the brake 48 is actuated causing thecaliper 49 to contact the disk 46. The resulting friction between thebrake 48 and the disk 46 causes the shaft to decrease in speed. Bylocating the brake in the recess 50 of the generator 20, room is savedin the nacelle 14 without risking contamination of the generator 20components. Potential contamination is further reduced by placing thisrecess on the down-wind side of the generator 20.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, may modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention.

1. A method for transferring electrical power from a wind turbinecomprising the steps of: rotating blades using wind; rotating agenerator; generating electricity with said generator; supporting saidgenerator with a tower; suspending a transformer adjacent saidgenerator; damping the movement of said tower by contacting saidtransformer; transmitting said electricity through said transformer. 2.The method of claim 1 wherein said damping is accomplished by contactingsaid transformer with said tower.
 3. The method of claim 1 wherein saiddamping is accomplished by contacting said transformer with a viscousfluid connected to said tower.
 4. The method of claim 1 furthercomprising the step of converting the frequency of said electricity. 5.The method of claim 4 further comprising the step of rotating said windturbine such that said blades are facing into the wind.
 6. The method ofclaim 5 further comprising the step of applying a brake to saidgenerator opposite said blades.
 7. A method for operating a wind turbinecomprising the steps of: rotating a plurality of blades; generatingvariable frequency electrical power by the rotation of said plurality ofblades; converting said variable electrical power from a firstelectrical characteristic to a second electrical characteristic;transferring said electrical power with said second electricalcharacteristic to a transformer suspended from a nacelle bedplate;damping the oscillations of said wind turbine with said transformer. 8.The method of claim 7 wherein said first electrical characteristic is afrequency between 10 and 30 Hz.
 9. The method of claim 8 wherein saidsecond electrical characteristic is a frequency between 50 and 60 Hz.10. The method of claim 8 wherein said oscillations are damped by thecontacting of said transformer against the wall of a tower.
 11. Themethod of claim 8 wherein said oscillations are damped by the movementof said transformer in a viscous fluid.
 12. The method of claim 9further comprising the step of transforming said variable electricalpower from an alternating current to a direct current.
 13. The method ofclaim 12 further comprising the step of transforming said direct currentto an alternating current.
 14. The method of claim 13 wherein saidtransformer is either a dry-type or oil-filled transformer.